Rupture geometry and multi-segment rupture of the …...India-Eurasia convergence (Avouac and Tapponnier, 1993; Tapponnier et al., 2001). The main trace of the Kunlun fault system

活断層・古地震研究報告，No. 4, p. 243-264, 2004

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Rupture geometry and multi-segment rupture of the November 2001 earthquake in the Kunlun fault system, northern Tibet, China

Bihong Fu1, Yasuo Awata2, Jianguo Du3 and Wengui He4

1, 2Active Fault Research Center, GSJ/AIST ([email protected], [email protected])3Institute of Earthquake Science, CEA ([email protected])

4Lanzhou Institute of Seismology, CEA ([email protected])

Abstract: We document the spatial distribution and geometry of surface rupture zone produced by the 2001 Mw 7.8 Kunlun earthquake, based on high-resolution satellite images combined with the field measurements. Our results show that the 2001 surface rupture zone can be divided into five segments according to the geometry of surface rupture. They are the Sun Lake, Buka Daban-Hongshui River, Kusai Lake, Hubei Peak and Kunlun Pass segments from west to east. These segments, varying from 55 to 130 km in length, are separated by step-overs or bends. The Sun Lake segment extends about 65 km with a strike of N45º-75ºW (between 90º05'E-90º50'E) along the previously unrecognized West Sun Lake fault. A gap about 30 km long exists between the Sun Lake and Buka Daban Peak where no obvious surface ruptures can be observed either from the satellite images or the field observations. The Buka Daban-Hongshui River, Kusai Lake, Hubei Peak and Kunlun Pass segments run about 365 km striking N75º-85ºW along the southern slope of the Kunlun Mountains (between 91º07'E-94º58'E). This segmentation of the 2001 surface rupture zone is well correlated with the pattern of slip distribution measured in the field; that is, the abrupt changes of slip distribution occur at segment boundaries. Detailed mapping suggest that these five first-order segments can be divided into over 20 second-order segments with the length of 10-30 km, linked by small-scale step-overs or bends. We re-measure the total rupture length produced by the 2001 Kunlun earthquake based on the high precision mapping. It shows that the total length is about 430 km (excluding the 30 km long gap), which is the longest among the intraplate earthquakes ever reported worldwide. On the basis of rupture geometry and segmentation, and slip distribution along the surface rupture zone, we suggest a multiple bilateral rupture propagation model. Our model shows that the faulting process of the 2001 Kunlun earthquake is quite complex, which consists of multiple westward and eastward rupture propagations, and interaction of these bilateral rupture processes. It is much more complex than the simple unilateral rupture process suggested by previous studies.

Keywords: satellite imagery; surface rupture; geometric feature; multi-segment earthquake; step-over and bend; strike-slip faulting; bilateral propagation; great intraplate earthquake; India-Eurasia collision; northern Tibet.

1. Introduction

Most large earthquakes in historical time ruptured only a part of long active faults (Ambraseys, 1970; Barka and Kadinsky-Cade, 1988; Stein et al., 1997). Surface ruptures often terminate at geometric or structural changes in the fault zone (Allen, 1968; Barka and Kadinsky-Cade, 1988). Changes in fault strength, geometry, and slip distribution may result in rupture segmentation (e.g., Ward, 1997; Zhang et al., 1999; Awata et al., 2001). This segmentation is manifested by consistent spatial and temporal rupture behavior that may be used to forecast the timing and magnitude of future events (e.g., Schwartz and Coppersmith, 1984; Van der Woerd et al., 1998).

The Kunlun earthquake, of moment magnitude (Mw) 7.8, occurred on 14 November 2001 along the Kusai Lake

fault of the Kunlun fault system, northern Tibet (Fig. 1; Lin et al., 2002). Immediately after the earthquake, several multidisciplinary research teams from China Seismological Bureau (CSB), and Institute of Geomechanics, Chinese Academy of Geological Sciences, and Shizuoka University (Japan) conducted field investigations on deformational characteristics of surface rupture zone and engineering damage. The preliminary results show that the Kunlun earthquake produced a long surface rupture zone with a length of ~350 km (CSB, 2002; 2003; Dang and Wang, 2002; Xu et al., 2002) or ~400 km (Lin et al., 2002; Fu and Lin, 2003). The typical lateral slip is 3 to 7 m (Xu et al., 2002; Fu et al., 2003; 2004a), and the largest one is up to 16.3 m (Lin et al., 2002). Field investigations also indicate that the surface rupture zone is composed of distinct shear faults, extensional cracks, mole tracks, and pull-apart sag ponds.

Bihong Fu, Yasuo Awata, Jianguo Du, Wengui He

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The width of surface rupture zone ranges from several meters to 1000 m, generally from 5 m to 50 m (Fu et al., 2003, 2004a). This earthquake also triggered avalanches of snow and glacial ice and caused slight damage to the base of the Qinghai-Tibet railway and road between Golmud and Lhasa. Ground shaking collapsed temporary housing for workers on the railway (Fu et al., 2003).

Up to now, there is no detailed report on the geometry of surface rupture zone. The surface rupture occurred on the Kunlun Mountains with an elevation over 4500 m, where detailed field mapping of the surface rupture is difficult. Thus, it is difficult to document the entire geometry of surface rupture zone from the field mapping.

In this study, we shall document geometric and geomorphic features of the 2001 Kunlun surface rupture zone based on detailed analysis of high-resolution satellite images combined with the data obtained from the field investigations. We shall also discuss rupture segmentation and rupture processes related to the 2001 seismic event.

2. Data and methods

Satellite remote sensing technique has been demonstrated as a powerful tool for mapping coseismic deformational features caused by large earthquakes (e.g., Fu and Lin, 2003; Wright et al., 2004). Satellite remote sensing data acquired by the American Landsat Thematic Mapper (TM) and Enhanced Thematic Mapper (ETM), the French Systeme Probatoire de 1’ Observation de 1a Terre (SPOT) High Resolution Visible (HRV) sensor, and the Japanese Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) currently provide important sources for earth surface mapping. Landsat TM/ETM have 15 to 30 m ground resolution, whereas ground resolutions of SPOT HRV Panchromatic (PAN) and ASTER data in Visible and Near Infrared (VNIR) region are 10 m and 15 m, respectively. Pre-earthquake (during 1998 to 2000) and post-earthquake (from late November, 2001 to May, 2002) satellite remote sensing data covering the western part of the Kunlun fault system are used in this study. The rupture zone can not be distinguished clearly from post-earthquake satellite images obtained after June 2002 because the surface rupture zone was eroded by summer snow and ice meltwaters. In addition, to analyze geometry of the 2001 surface rupture in detail we use post-earthquake American IKONOS data with 1 m resolution for post-earthquake period for some key regions.

These multi-source satellite remote sensing data were geometrically corrected and processed using ER-Mapper. Image mosaic, contrast stretch and band composition were used to enhance imagery feature of the surface rupture zone. Above-mentioned satellite remote sensing data provide an excellent view of the 2001 Kunlun earthquake surface rupture.

Satellite images at different scales (1/5,000 to 1/100,000) are used to interpret the geometric and geomorphic features of the 2001 rupture zone. Moreover, we carried out the field investigation several times along

the surface rupture zone to obtain detailed deformational and geometric features of the surface rupture zone.

3. Late Quaternary activity and seismicity of the Kunlun fault system

The Tibetan Plateau is one of the most imposing topographic features on the earth surface (Fig. 1a), having a mean elevation of ~4500 m, which was resulted from India-Eurasia collision (Molnar and Tapponnier, 1975; 1978; Peltzer et al., 1989). The E-W to WNW-ESE striking Kunlun fault system, extending about 1600 km between 86ºE to 105ºE, is one of the largest strike-slip faults in the northern Tibet, China (Molnar and Tapponnier, 1975; Van der Woerd et al., 1998; SBPQ, 1999). As a major strike-slip fault, it plays an important role in the eastward extrusion of Tibet plateau accommodate to northeastward shortening caused by the India-Eurasia convergence (Avouac and Tapponnier, 1993; Tapponnier et al., 2001). The main trace of the Kunlun fault system may be roughly divided into six principal faults, each 150-270 km long, on basis of the geometry of fault trace and structural assemblage (Fig. 1b; Van der Woerd et al., 1998). The left-lateral strike-slip faulting of the Kunlun fault may have initiated since the early Pliocene or early Pleistocene (Kidd and Molnar, 1988; Fu and Awata, 2004). Analyses of satellite images, cosmogenic surface dating and radiocarbon dating and trench surveys suggested that the Kunlun fault is an active left-lateral strike-slip fault with an average slip rate of 10-20 mm/yr in the Quaternary (Kidd and Molnar, 1988); or 11.5 ± 2 mm/yr in the past 40 kyr (Zhao, 1996; Van der Woerd et al., 1998; 2000).

Three large earthquakes (M > 7.0) including the 14 November 2001 Mw 7.8 Kunlun earthquake occurred along the Kunlun fault system during the last 100 years. The 18 November 1997 Manyi earthquake with Mw 7.6 broke the western-most 170-km-long Manyi fault between 86º30’E and 88º30’E (Fig. 1b). Maximum left-lateral displacement is ~7 m for the Manyi earthquake as revealed by synthetic aperture radar (SAR) interferometry (Peltzer et al., 1999). The 1937 Ms 7.5 Tuosuo Lake earthquake produced rupture zone about 300-km-long between 96ºE and 99ºE on the Tuosuo Lake fault with a maximum left-lateral offset of 6-7 m (Jia et al., 1988; Fig. 1b). No historical record of earthquakes had been reported along the Kusai Lake and Xidatan-Dongdatan faults although there are geomorphic features associated with seismic activities (Kidd and Molnar, 1988; Van der Woerd et al., 1998; 2000). Based on the trenching survey, it has been inferred that recurrence interval of great earthquakes in the Xidatan-Dongdatan fault is 850 ± 200 yr with a characteristic slip of 10 ± 2 m (Zhao, 1996; Van der Woerd et al., 1998; 2000; 2002).

4. Geometry and segmentation of the 2001 surface rupture zone

In the satellite image, the trace of Kusai Lake and

Rupture geometry and multi-segment rupture of the November 2001 earthquake in the Kunlun fault system, northern Tibet, ChinaRupture geometry and multi-segment rupture of the November 2001 earthquake in the Kunlun fault system, northern Tibet, China

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Kunlun Pass faults exhibit striking lineaments (Fig. 2a). The 2001 surface rupture zone distributes mostly along the pre-existing Kusai Lake and Kunlun Pass faults. We can divide the surface rupture zone into five first-order segments according to their geometric and structural features (Fig. 2b). They are the Sun Lake, Buka Daban-Hongshui River, Kusai Lake, Hubei Peak, and Kunlaun Pass segments from west to east, respectively. These first order segments, each 55 to 130 km long, are linked by 5 to 30 km long and 1 to 10 km wide step-overs or bends (Figs. 3, 6, 8a, 9, 10a, 12, 13). The detailed geometry of each segment is summarized as follows.

4.1 The Sun Lake SegmentThe Sun Lake segment extends ~65 km between the

Kushuiwan Lake (90º05’E) and east bank of the Sun Lake (90º50’E, Fig. 3). This segment can be further divided into four second-order segments, each 10 to 20 km long (Fig. 3b). It is the westernmost part of the 2001 November rupture zone (Zhao et al., 2002) and strikes N40º-75ºW (Fig. 3). Typical lateral offset is 1 to 3 m as observed in the field (Zhao et al., 2002). At the westernmost termination of the Sun Lake segment, some N70º-75ºW striking fissures and a N10ºW extending pressure ridge occurred in the frozen surface of the Kushuiwan Lake (Fig. 4a). On the east bank of the Kushuiwan Lake, two distinctive surface ruptures appear right-stepping pattern (Fig. 4a). The ruptures show a right-stepping en echelon arrangement in the central part of the Sun Lake segment. Eastward to the south of Peace Island, rupture zone splays into two distinctive ruptures (Fig. 3b). The southern branch runs about 3 km striking N45ºW, and northern branch continuously extends towards the Sun Lake with a strike of N65ºW (Figs. 4b and 5a). Large-scale IKONOS image shows that the third-order rupture segments (a, b, c, d, e, f, and g in Fig. 5a), each several hundreds meters to 2 km long, display a typical right-stepping en echelon pattern (Fig. 5a). In the field, the striking rupture zone occurs on the late Quaternary alluvial fans (Fig. 5b). Farther east to the west of the Sun Lake, three major ruptures developed in the frozen surface of the Sun Lake, which display as a tail pattern (Figs. 3b and 4b). These ruptures terminate on the eastern bank of the Sun Lake near 90º50’E (Fig. 3b). There is an about 30 km long gap between the Sun Lake and Buka Daban-Hongshui River segments, which display a left-stepping pattern (Figs. 2b and 3b). No obvious surface ruptures can be observed from the satellite images or field investigations. It may be a large unbroken barrier between the Sun Lake and Buka Daban-Honshui River rupture segments, where the step-over shows a large pull-apart basin (Fig. 3).

4.2 The Buka Daban -Hongshui River SegmentThe trace of the Buka Daban-Hongshui River segment

runs 110 km between the southeast Buka Daban Peak (91º07’E) and Hongshui River (92º18’E) with a strike of N75º-80ºW (Fig. 6). In the westernmost part of this segment, the rupture zone trends N45ºE and three left-stepping en-echelon arranging ruptures display a horse-

tail pattern occurred on the moraine deposits on the southeast of the Buka Daban Peak (Figs. 6b and 7a). The IKONOS image exhibits the left-stepping geometry of major rupture zone, and the stream across the rupture zone is displaced left-laterally (Fig. 7b). Northeastward, the rupture bends into N80ºW (Fig. 6b). The Buka Daban-Hongshui River segment is further divided into 5 to 6 second-order segments, each 20-30 km long, interconnected by small-scale bends or step-overs (Fig. 6b). These small bends or step-overs, 1-3 km long and several hundreds meters wide, appear as the pressure-ridges on satellite image (Fig. 6a).

4.3 The Kusai Lake SegmentTo the east of the Hongshui River, the rupture zone is

called the Kusai Lake segment (Fig. 2b). There is a large step-over, ~10 km long and 1 km wide, between the Buka Daban-Hongshui River and Kusai Lake segments (Figs. 6b and 8a). The IKONOS image clearly shows that the ruptures consist of third-order segments, each several hundreds meters long, which are linked by small step-overs or bends (Fig. 8b). The pressure ridges are formed in these small-scale bends or step-overs (Fig. 8b). The Kusai Lake segment extends 55 km between 92º15’E and 92º48’E. This segment also can be further divided into five second-order segments by small-scale step-overs or bends (Fig. 9b). There are large-scale half-graben and pressure ridges, several kilometers long and several hundreds meters wide, linking these second-order segments (Fig. 9b). A segment boundary is just located on the northeast of Kusai Lake (Fig. 9b), where the ruptures display a complex geometry, appearing flat-lying Y-shape pattern (Fig. 10a). In the field, a striking rupture zone passes through northeast of the Kusai Lake (Fig. 10b).

4.4 The Hubei Peak SegmentThe Hubei Peak segment distributes to the south of the

Kunlun range, where several ice-capped high peaks stand over 5500 m. The length of this segment is about 70 km with a strike of N75º-85ºW between 92º48’E and 93º40’E (Fig. 9b). The rupture zone cut the pre-Quaternary basement rocks and Quaternary fluvial fans (Fig. 9b). Again, the Hubei Peak segment is divided into 5 to 6 second-order segments, each 10 to 30 km long (Fig. 9b). The small step-overs and bends, 1-3 km long and several hundreds meters wide, link those second-order segments (Fig. 11). To the east of this segment, the rupture zone consists of several sub-parallel ruptures with several hundreds meters separation (Fig. 9b), where the widest surface deformation zone was observed in the field (Lin et al., 2002; Xu et al., 2002). To the east of 93º30’E, the rupture zone splays into two branches: the northern one extends along the pre-existing Xidatan-Dongtatan fault of the Kunlun fault system, and it terminates near 93º40’E, whereas the southern branch continuously runs eastward (Fig. 9b). These two splaying ruptures appear as a left step-over, which forms a boundary between the Hubei Peak and Kunlun Pass segments (Figs. 12 and 13).


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4.5 The Kunlun Pass SegmentThe trace of the Kunlun Pass segment runs mostly

along the pre-existing Kunlun Pass fault, extending more than 130 km in length between 93º35’E and 94º58’E (Fig. 13). This segment can be also separated into 6 to 7 second-order segments, 10-30 km long (Fig. 13 b). The surface rupture starts at south of 5933 m high peak, passes through the north of the Kunlun Pass, and continues to farther east. On the southern slope of Yuzhu Peak, the rupture zone runs across the modern glaciers (Fig. 13). Detailed geometry of the eastern part of the Kunlun Pass segment can be clearly observed in the large-scale post-earthquake SPOT image (Fig. 14a). Near the easternmost end of this segment, the rupture zone splays into two major zones at point B, which display a flat-lying Y-shape pattern as shown on the SPOT image (Fig. 14a). The northern one extends straightly eastward, and terminates near 94º55’E (point C in Fig. 14a). The southern one bends southeastward and appears as a horse-tail pattern (Fig. 14a). Finally, the surface rupture zone disappears near 94º58’E (point E in Fig. 14a). At the easternmost end of the Kunlun Pass segment, the ruptures consist of the right-stepping cracks with a strike of N60º-65ºE as observed in the field (Fig. 14b).

5. Discussion

5.1 Linkage among segment geometry, slip distribution and rupture process

It is a timely topic how to link the relations among segment geometry, surface displacements, and rupture propagation process of the long seismogenic faults (e.g., Schultz, 1999; Phillips et al., 2003). Our results show that the surface rupture zone, produced by the 2001 Mw 7.8 Kunlun earthquake, has a multi-segment pattern as described in the section 4 (Fig. 15a). The surface rupture zone is geometrically segmented at a variety of scales (Figs. 2b, 3b, 6b, 9b, and 13b). It can be divided into five first-order segments, individual segment length varying 55 km to 130 km. These first-order segments are separated by the step-overs 5-30 km long and 1-10 km wide or bends (Fig. 15a). Meanwhile, these first-order surface rupture segments are subdivided into over 20 second-order segments of 10 to 30 km long by smaller step-overs or bends (Figs. 3, 6, 9 and 13). Moreover, even smaller-scale third-order segments, each several hundreds meters to 3 km long, can be identified either from high-resolution satellite images (Figs. 5a, 7b and 8b) or field observations (Fu et al., 2003). A number of studies have demonstrated that faults are geometrically and mechanically segmented at variety of scales (e.g., Allen, 1968; Allen et al., 1991; Wallace, 1970; 1990). For example, the number of segments identified for the San Andreas fault ranges from 4 to 984 (Allen, 1968; Wallace, 1970). As explained by Schwartz and Sibson (1989), segments may represent the repeated coseismic rupture during a single event on a long fault with tens to hundreds of kilometers, or they may represent a part of a rupture associated with an individual faulting event and only a

few kilometers long. Similarly, the surface rupture segments with different scales might be products of rupture events or sub-events with different scales (e.g., DePolo et al., 1991; Awata et al., 2001).

Our mapping also exhibits that step-overs or bends with different scales exist at segment boundaries (Figs. 3b, 4, 5a, 6b, 8, 9b, 10a, 11, 12 and 13b). These step-overs and bends are very important features that control the process of rupture initiation, propagation and termination (e.g., Segall and Pollard, 1980; King and Nabelek, 1985; Zhang et al., 1999). They not only reflect structural and geomorphic changes of fault geometry on the surface, but also represent variations of mechanical fault movement at depth (e.g., Schwartz and Coppersmith, 1984; Ward, 1997; Sugiyama et al., 2002). Formation mechanism of step-over and bend and their geomorphic features had been noted by a number of previous studies (e.g., King, 1986; Dooley and McClay, 1997; McClay and Bonora, 2001). At these segment boundaries, the slip distribution along the rupture zone shows an abrupt change (Fig. 15b). Thus, multi-segment geometry pattern of the surface rupture zone might record the processes of fault nucleation, slip, linkage and propagation (Schultz, 1999; Aydin and Kalafat, 2002).

Comparing the analysis of geometry of the surface rupture zone and pattern of slip distribution with the seismic data, we suggest a model to explain the faulting processes or rupture propagation during the 2001 November earthquake (Fig. 15c). As indicated by seismic waveform inversion, the 2001 Mw 7.8 Kunlun earthquake may consist of three major sub-events (Lin et al., 2003; Xu and Chen, 2004). Our model shows that the 2001 Kunlun seismic event has a complex multiple rupture process (Fig. 15c). In our model, the rupture started near 90º30’E, which is consistent with the epicenter location reported by the USGS (Fig. 15b), and it propagated bilaterally eastward and westward. Westward rupture terminated on the Kushuiwan Lake near 90º05’E (Fig. 15c). The seismic moment of first sub-event is equal to an Mw 6.9 shock as revealed by seismological data (Antolik et al., 2004). Eastward rupture passed through the Sun Lake, and then jumped onto the main trace of the Kusai Lake fault at 91º07’E in the southeast of the Buka Daban Peak (Figs. 7a and 15c). The second sub-event occurred near 92º15’E, which is similar with location of Harvard Moment Centriod (Bufe, 2004). The second sub-event is a major rupture event, which is equal to an Mw 7.8 event (Antolik et al., 2004). The rupture from the second sub-event continuously propagated eastward and passed through the Kusai Lake and south slope of the Kunlun Range. It triggered the third sub-event near 93º00’E (Fig. 15c), which is the same with the location of third sub-event suggested by Xu and Chen (2004). The seismic moment of third sub-event is equal to an Mw 6.7 event (Antolik et al., 2004). Then, it continuously propagated eastward along the Kunlun Pass fault, but the fracture energy gradually dropped. Finally, it terminated near 94º58’E (Fig. 15c). Temporally, the second and third sub-events occurred 52 and 56 seconds, respectively, after the


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first sub-event as revealed by seismological data (Xu and Chen, 2004). However, our model is different from the unilateral rupture process inferred from the inversion of seismic data (Lin et al., 2003; Antolik et al., 2004). Our model suggests that rupture caused by the second sub-event also propagated westward. At the east of the Buka Daban Peak, the rupture propagated southwestward, and terminated near 91º07’E (Figs. 3b and 15c). It can reasonably explain the complex geometry pattern of surface rupture zone in the southeast of the Buka Daban peak (Fig. 7). It should be noted that a 30 km long rupture gap exists between the Sun Lake and Buka Daban-Hongshui River segments (Fig. 3b). It probably is an unbroken barrier (Aki, 1984).

5.2 Length of the surface rupture It remains a debate about the length of the 2001 Kunlun

surface rupture zone. For example, Xu et al. (2002) reported a ~350 km long rupture zone extending between 91º07’E-94º48’E. The high-precision mapping of satellite images clearly indicate that the five first-order rupture segments are 65 km, 110 km, 55 km, 70 km and 130 km for the Sun Lake, Buka Daban-Hongshui River, Kusai Lake, Hubei Peak and Kunlun Pass segments, respectively (Figs. 3b, 6b, 9b and 13b). Thus, the total length of the surface rupture zone should be ~430 km (excluding a 30 km long gap). This spatial distribution of surface rupture zone has also been confirmed by the field observations (Figs. 5b, 10b and 14b; Zhao et al., 2002; Fu et al., 2004a), and it is also consistent with the distribution of aftershocks (Fig. 15a). The previous reported longest surface rupture zone is 375 km long, which was produced by 1905 M 8.2 Bulnay, Mongolia, earthquake (Yeats et al., 1997). The surface rupture zone produced by the 2001 Mw 7.8 Kunlun earthquake is, therefore, the longest rupture zone produced by a single intraplate earthquake ever reported worldwide.

5.3 Implications for future earthquakes along the Kunlun fault

The previous studies have demonstrated that the segment boundaries linking the fault segments are unstable features and earthquake can easily occur near the segment boundary (e.g., King and Nabelek, 1985; Fu et al., 2004b). Therefore, it should be noted that the 2001 Kunlun earthquake did not rupture the Xidatan-Dongdatan segment of the Kunlun fault system (Figs. 13b and 15a). It is a “seismic gap” between the 1937 Ms 7.5 Tuosuo Lake earthquake and the 2001 Mw 7.8 Kunlun earthquake (Fig. 1b). Tectono-geomorphic feature shows that it is a large pull-apart basin, in which a large earthquake event may occur in the future. In the Xidatan area, engineering design of the Qinghai-Tibet railway under construction thus should consider the future possible large earthquake. The special engineering design of the Trans-Alaska oil pipeline, which did not break during the 2002 Mw 7.9 Denali Fault earthquake, provides a successful example (Phillips et al., 2003).

6. Conclusions

Based on the detailed analyses of satellite images and field observations, we can conclude as follows:

(1) The 2001 November surface rupture zone can be divided into five first-order segments according to the geometry of surface rupture zone. These first-order segments, each 55 to 130 km long, are linked by large step-overs and bends. These first-order segments are further divided into over 20 second-order segments, each 10 to 30 km long, by smaller step-overs and bends.

(2) Rupture propagation process associated with the 2001 Kunlun earthquake is quite complex. It suggested that multi-segment geometry of the 2001 Kunlun rupture zone is a product of the multiple westward and eastward rupture propagation, and interaction of these bilateral rupture propagation, which is consistent with rupture process of three major sub-events suggested by seismological research.

(3) High precision measurement shows that total length of surface rupture zone is ~430 km, which is the longest one among the intraplate earthquakes ever reported worldwide.

In general, the analyses of the 2001 surface rupture zone provide new insights into the complex multi-segment geometry and multiple bilateral rupture propagation associated with a great intraplate earthquake along a long strike-slip fault.

Acknowledgements

This study benefited from discussion with A. Lin, K. Kano, E. Tsukuda, Y. Sugiyama, X. Lei, M. Saito and H. Sekiguchi. We thank K. Satake for his constructive comments and careful review, which are helpful to improve this manuscript. ASTER data were provided by the Earth Remote Sensing Data Analysis Center (ERSDAC), Japan. We also used the DEM data released by USGS Shuttle Radar Topography Mission (SRTM) Project in Fig. 1a. Especially, we are grateful to Seismological Bureau of Xinjiang for the permission to use one photograph (Fig. 5b). B. Fu would like to acknowledge the support from Postdoctoral Fellowship (P 03510) of Japan Society for the Promotion of Science (JSPS).

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Peltzer, G., Grampe, F., King, G. (1999) Evidence of nonlinear elasticity of the crust from the Mw 7.6 Manyi earthquake. Science, 286, 272-276.

Schultz, R.A. (1999) Understanding the process of


249

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(Received 19 August 2004, Accepted 5 October 2004)


250


104

Eo10

4Eo

102o

102o

100o

100o

98o

98o

96o

96o

94o

94o

92o

92o

90o

90o

88o

88o

86o

86o

36o

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36o

38o

Fig.2

Fig.2

Gol

mud

Go

lmu

d

Kun

lun

Pas

sK

unlu

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aD

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k(6

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k(6

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iver

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ount

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ault

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ault

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ake

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ake

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in

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in

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ount

ains

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iver

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ake

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alin

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ake

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ake

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ake

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oL

ake

0240km

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-Ter

tiary

rock

s

Qua

tern

ary

toTe

rtia

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cks

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ive

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ak

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am

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ult

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cent

er

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Mw7.6

Mw7.6

Nov.08,1997

Nov.08,1997

Mw7.8

Nov.14,2001

Mw7.8

Nov.14,2001

Ms7.5

Jan.07,1937

Ms7.5

Jan.07,1937

b

70Eo

80Eo

100

Eo

30N

o

90Eo

20No

40No

70Eo

80Eo

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Eo

30N

o

90Eo

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120

Eo11

0E

o12

0Eo

TIBET

TIBET

INDIA

INDIATARIM

TARIM

TIANSHAN

TIANSHAN

500

0km

Fig

.1b

Fig

.1b

a

Fig.

1.(a

)Sh

aded

-rel

iefim

age

ofth

eIn

dia-

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aco

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ated

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ely.

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rof

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rmin

edby

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.Geo

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calS

urve

y(U

SGS)

.


251

Pre-Tertiaryrocks

Quaternary andTertiary rocks

Rupture zone Ice-cap LakeActive strike-slip fault

SunLakeSunLake

KekexiliLake

KekexiliLake

Hon

gshu

i River

Hon

gshu

i River

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Fig. 3Fig. 3

Fig. 6Fig. 6

Fig. 9Fig. 9

Fig. 13Fig. 13

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KunlunPass

KunlunPass

6860 m6860 m

6056 m6056 m5805 m5805 m 5863 m5863 m

6178 m6178 m

XidatanXidatan

0 100 km

SL Seg.SL Seg.HB Seg.HB Seg.

KP Seg.KP Seg.

KL Seg.KL Seg.

BK-HR Seg.BK-HR Seg.

b NN

92o90o 94 Eo92o90o 94 Eo

36oN36oN

GolmudGolmud

KunlunPassKunlunPass

Buka DabanPeak

Buka DabanPeak

6860m

ERIUSGS &

a NN

Fig. 2. (a) Landsat ETM mosaic image of the Kusai Lake and Kunlun Pass faults of the Kunlun fault system. Fault traces indicated by arrows.The epicenter and focal mechanism of the 2001 Mw 7.8 Kunlun earthquake were determined by the USGS and Earthquake ResearchInstitute (ERI) of the University of Tokyo, respectively. (b) Spatial distribution of the surface rupture zone produced by the 2001Kunlun earthquake (Modified after Fu and Lin, 2003). SL Seg.: Sun Lake segment; BK-HR Seg.: Buka Daban-Hongshui Riversegment; KL Seg.: Kusai Lake segment; HB Seg.: Hubei Peak segment; KP Seg.: Kunlun Pass segment.


252


020

km

Fig

.4a

Fig

.4a

Fig

.7a

Fig

.7a

Fig

.4b

Fig

.4b Su

nL

ake

6860

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60m

Bu

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aban

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kB

uka

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eak

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ush

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ake

9100'

o90

30'

o

9000'E

o

9100'

o90

30'

o

9000'E

o

3600'N

o36

00'N

oaN N

SL

Seg

.S

LS

eg.

BK

-HR

Seg

.B

K-H

RS

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Gap

4981

m49

81m

6860

m68

60m

Bu

kaD

aban

Pea

k

6004

m60

04m

Pea

ceIs

land

Pea

ceIs

land

9100'

o90

30'

o

9000'E

o

o

9100'

o90

30'

o

9000'E

o

3600'N

o36

00'N

o

Su

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ake

Su

nL

ake

Ku

shu

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Lak

e

Ku

shu

iwan

Lak

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020

km

(US

GS

)(U

SG

S)

Pre-Quaternary

rocks

Quaternary

sediments

Activefault

Snow&glacier

Rupture

zone

Stream

channel

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Lake

Boundaryof

firstand

second-order

segm

ent

bNN

Fig.

3.(a

)L

ands

atE

TM

imag

eof

the

Sun

Lak

ese

gmen

t(A

ugus

t16

,200

2).(

b)In

terp

reta

tive

map

show

ing

geom

etry

ofth

ew

est

part

ofsu

rfac

eru

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ezo

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ote

that

ther

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km-l

ong

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een

90°5

0'E

and

91°0

7'E

indi

cate

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iang

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For

loca

tion

see

Fig.

2b.


253

0 5 km0 5 km

Fig.5aFig.5a

Fig.5bFig.5b

b

0 5 km0 5 km

NNa

Fig. 4. Post earthquake (January 05, 2002) ASTER images exhibiting geometry of the surface rupture zone. (a) Rupturesappearing as right-stepping en-echelon pattern in the westernmost termination. (b) Ruptures displaying a tail pattern inthe Sun Lake. Locations of Figs. 4a and 4b are shown on Fig. 3a.


254


Sun LakeSun Lake

b SESSES

0 1 km0 1 km

ab

cd

e

f

NNa

Fig. 5. (a) IKONOS image (September 24, 2002) showing detailed features of surface ruptures to west of the Sun Lake. Forlocation see Fig. 4b. (b) Photograph showing a distinctive rupture in west bank of the Sun Lake (provided bySeismological Bureau of Xinjiang, CSB) .


255

3600'N

o

9200'

o92

00'

o92

00'

o91

30'E

o91

30'E

o

9100'

o91

00'

o

3600'N

o

020

km

Pre-Quaternary

rocks

Quaternary

sediments

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zone

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ent

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.

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-HR

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o

o

o92

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00'E

o

3600'N

o

9200'

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00'

o

3600'N

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3550'

o35

50'

o

020

km

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.7a

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.7a

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6.(a)Sa

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theBuk

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ongs

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ent.(b

)In

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etry

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.


256


o

Fig.7bFig.7b

MoraineMoraine

91 10'Eo91 10'Eo91 05'o91 05'o

36 00'No36 00'No

1 km0 1 km0

NNa

0 50 m

NNb

Fig. 7. (a) Post-earthquake (December 26, 2001) SPOT image showing tail pattern of surface rupture in the west end of BukaDaban-Hongshui River segment. For location see Fig. 3a or Fig. 6a. (b) IKONOS image (September 24, 2002) showingthat the surface ruptures appear a right-stepping pattern in southeast of the Buka Daban Peak. For location see Fig. 7a.


257

0 500 mPRPR

PRPR

NNb

Fig.8bFig.8b

0 2 km

River

Hongshui

River

Hongshui

NNa

Fig. 8. (a) IKONOS image (January 9, 2002) showing that the segment boundary between the Buka Daban-Hongshui Riverand Kusai Lake segments appears as a step-over, 10 km long and 500 m wide. (b) Enlarged IKONOS image showingthe detailed geometric and geomorphic features of the ruptures. Note that the ruptures are separated by third-orderstep-overs or bends. PR: pressure ridge.


258


9300

'o

9300

'o

9230

'Eo

9230

'Eo

9330

'o

9330

'o

3540

'o

3540

'o

3550

'No

3550

'No

020

km20

km

5646

m56

46m

5769

m57

69m

Pea

kH

ubei

Pea

kH

ubei

Kus

ai

Lake

Kus

ai

Lake

Pre

-Qua

tern

ary

rock

s

Qua

tern

ary

sedi

men

ts

Nor

mal

faul

t

Sno

w&

glac

ier

Rup

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Strea

mch

anne

l

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scar

p

Lake

Bou

ndar

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first

and

seco

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segm

ent

NNb

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.H

BS

eg.

KL

Seg

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9300

'o

9300

'o

9230

'Eo

9230

'Eo

9330

'o

9330

'o

3540

'o

3540

'o

3550

'No

3550

'No

020

km20

km

NNa

Fig

.10a

Fig

.10a

Fig

.12a

Fig

.12a

Fig

.11a

Fig

.11a

Fig

.11b

Fig

.11b

Fig.

9.(a)Sa

telli

temos

aicim

agesh

owingge

omor

phic

featur

esof

theKus

aiLak

ean

dHub

eiPe

akse

gmen

ts.(

b)In

terp

retativ

em

apex

hibi

tingge

ometry

ofth

eKus

aiLak

ean

dHub

eiPe

akse

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tsof

the20

01co

seismic

surfac

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ptur

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ne.F

orloca

tionse

eFi

g.2b

.


259

SS

Kusai LakeKusai Lake

b

Fig.10bFig.10b

NNa

KusaiLake

0 5 km

Fig. 10. (a) ASTER VNIR image (December 6, 2001) showing distinctive coseismic ruptures in the north of the Kusai Lake.For location see Fig. 9a. (b) Photograph showing the surface rupture zone in the northern bank of Kusai Lake. Forlocation see Fig. 10a.


260


0 1 km0 1 km

BendBend

NNb

0 1 km0 1 km

Step-overStep-over

NNa

Fig. 11. (a) ASTER VNIR image (November 29, 2001) showing a right-stepping en-echelon pattern of the ruptures. Note thata pressure ridge formed between two ruptures in the northeast of the Kusai Lake. (b) SPOT image exhibiting a bendbetween two second-order segments in the Hubei Peak segment. For locations see Fig. 9a.


261

5933 m5559 m

Pre-Quaternaryrocks

Quaternarysediments

Normal fault

Snow & glacierRupture zone Streamchannel

Avalanch of snowand glacier triggered byearthquake

0 5 km

bN

0 5 km

a N

Fig. 12. (a) SPOT image (January 16, 2002) showing the segment boundary between the Hubei Peak and KunlunPass segments. The segment boundary appears as a step-over, where the ruptures display a complicated pattern.(b) Interpretative map displaying the step-over between two segments. For location see Fig. 9a.


262


3540

'o

3530

'No

9400

'Eo

9430

'o

3540

'o

3530

'No

9500

'o

6178

m61

78m

5590

m55

90m

Ku

nlu

nP

ass

Ku

nlu

nP

ass

5933

m59

33m

Yuz

huP

eak

Yuz

huP

eak

Xid

atan

Xid

atan

-Do

ng

dat

anF

ault

Xid

atan

-Do

ng

dat

anF

ault

KP

Seg

.

020

km

bN

Pre

-Qua

tern

ary

rock

s

Qua

tern

ary

sedi

men

ts

Act

ive

strik

e-sl

ipfa

ult

Sno

w&

glac

ier

Rup

ture

zone

Strea

mch

anne

l

Bou

ndar

yof

first

and

seco

nd-o

rder

segm

ent

o

o

Ku

nlu

nP

ass

Ku

nlu

nP

ass

Xid

atan

3530

'No

9400

'Eo

9430

'o

9430

'o

3530

'No

9500

'o

3540

'o

3530

'No

9400

'Eo

9430

'o

9430

'o

3530

'No

9500

'o

3540

'o

020

km

Na

Fig

.14a

Fig

.14a

Fig.13.(a)

Satellitemosaicim

ageexhibitin

ggeom

orphicfeatures

oftheKunlunPass

segm

ent.(b)Interpretativ

emap

show

inggeom

etry

oftheKunlunPass

segm

ent.Fo

rlocatio

nseeFig.2b.


263

SWb

AA

CC

BB

DD

EE

0 2 km

NNa

Fig.14. (a) SPOT image (January 25, 2002) showing that the easternmost termination of the 2001 surface rupture zone,appearing as a flat-lying Y-shape pattern. Location is shown on Fig. 13a. (b) Photograph showing that rupture zonegradually disappeared near the easternmost end, where the ruptures appearing an en-echelon pattern. For locationsee point E in Fig. 14a.


264

Horizo

nta

ldis

pla

cem

ent(m

)

55

90 00'Eo

90 30'o

91 00'o

91 30'o 92 00'

o92 30'

o93 00'

o94 00'

o95 00'

o93 30'

o94 30'

o

92 00'Eo

94 00'o

92 00'Eo

94 00'o

36 00'No

36 00'No

SunLakeSunLake

KekexiliLake

Hongshui

HongshuiRiver

KunlunPass

KunlunPass

KusaiLakeKusaiLake

(6860 m)

6178 m6178 m

Buka DabanPeak

5933 m5769 m

5863 m5863 m5805 m

6004 m

0 100 kmaftershock

Seg.bound

arySeg. boundary

Seg. boundarySeg. boundary

Xidatan-Dongdatanfault

Xidatan-DongdatanXidatan-Dongdatan fault

b

c

a

Fig.15. (a) Geometry and segmentation of the surface rupture zone associated with the 2001 large seismic event. Thedistribution of major aftershocks (ML>3.0) occurred during 14 November 2001 and 27 November 2001(modified from Xu and Chen, 2004). (b) Slip distribution along the surface rupture zone, compiled from ourfield measurements and previous published results (CSB, 2002; Zhao et al., 2002). Locations of three majorsub-events are same with the USGS`s epicenter, Harvard Moment Centriod, and third sub-event suggested byXu and Chen (2004), respectively. (c) Rupture propagation model showing a complex multiple ruptureprocess related to the 2001 seismic event.

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Rupture geometry and multi-segment rupture of the …...India-Eurasia convergence (Avouac and Tapponnier, 1993; Tapponnier et al., 2001). The main trace of the Kunlun fault system